This invention relates to an exhaust gas recirculation control system for internal combustion engines, and more particularly to an automatic compensating device for automatically correcting a detected zero opening value of the exhaust gas recirculation valve to ensure accurate exhaust gas recirculation control.
A fuel supply control system is conventionally known which is adapted particularly for use with a gasoline engine and in which a fuel-injection type fuel quantity adjusting means for supplying fuel to the engine has its valve opening period electronically controlled so as to control the air/fuel ratio of an air/fuel mixture being supplied to the engine to a predetermined value.
As one of such fuel supply control systems there has been proposed e.g. by U.S. Pat. No. 3,483,851 a system which is adapted to determine the valve opening period of the fuel quantity adjusting means by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient conecentration (oxygen concentration), etc.
On the other hand, in an engine provided with an exhaust gas recirculation system (EGR), it is necessary to control the valve opening period of the fuel quantity adjusting means to different values between when the exhaust gas recirculation system is operating and when the same system is at rest. To put into practice such exhaust gas recirculation control, two different maps can be provided, for instance, which comprise a plurality of predetermined basic values of the valve opening period of the fuel quantity adjusting means, which are different from each other, and which are functions of different combinations of engine rpm and intake pipe absolute pressure. Selection of the two maps depends upon whether or not the exhaust gas recirculation system is operating. In carrying out exhaust gas recirculation control by using such two different maps, determination as to whether or not the exhaust gas recirculation system is operating can be made by comparing between an actual valve opening (valve lift) value of the exhaust gas recirculation valve and a valve opening (valve lift) command value selected from a map of required valve opening or lift values as a function of engine rpm and intake pipe absolute pressure. The above actual valve opening or lift value can be detected by a suitable detecting device, for instance, a lift sensor formed of a potentiometer which is connected to the valve body of the exhaust gas recirculation valve.
However, there can be a change in the reference zero position of the exhaust gas recirculation valve, that is, the valve opening value detected by the lift sensor when the valve body is in its full closing position, due to thermal expansion and wear of the component parts of the valve, etc. This causes a difference between an actual valve opening value and a valve opening value detected by the lift sensor or the like so that, for instance, even if the actual valve opening value is zero, the output of the lift sensor or the like shows a detected value other than zero. Therefore, it is impossible to accurately control the valve opening period of the fuel quantity adjusting means which should be controlled to values different between when the exhaust gas recirculation system is operating and when it is not operating, resulting in inaccuracy of the fuel supply control. Moreover, the above phenomenon also prevents accurate exhaust gas recirculation control which is carried out on the basis of the valve opening or lift command value.